System and method for reducing call dropping rates in a multi-beam communication system

ABSTRACT

A method for reducing call dropping rates in a multi-beam communication system. The multi-beam communication system includes a user terminal, a gateway, and a plurality of beam sources, where each beam source projects a plurality of beams, and where a communication link between the user terminal and the gateway is established on one or more beams. The method according to the present invention relies on a messaging protocol between the gateway and the user terminal. Based on messages sent from the user terminal to the gateway, preferably on a preselected periodic basis, the gateway can determine the more desirable beam(s) for transmitting data or information to the user terminal. The messages sent from the user terminal to the gateway contain values representing beam strengths as measured at the user terminal. The gateway uses the user terminal measured beam strengths to select the beams that should be used for transmitting data or information to the user terminal. The beams that should be used are the beams that if used will decrease the call dropping rates and provide a desired level of beam source diversity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is related to the following commonly owned,co-pending U.S. utility patent application Ser. No. 08/722,330, filed onSep. 27, 1996, entitled “Method and Apparatus for Adjacent Service AreaHandoff in Communication Systems,” which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to the field of wirelesscommunications. More specifically, the present invention relates to amethod for reducing call dropping rates in a wireless communicationsystem having multiple beam communication links.

II. Related Art

There are a variety of wireless communication systems having multiplebeam communication links. A satellite-based communication system is onesuch example. Another example is a cellular communication system. Asatellite-based communication system includes one or more satellites torelay communications signals between gateways (also referred to as“communication stations” or “base stations”) and user terminals.Gateways provide communication links for connecting a user terminal toother user terminals or users of other communications systems, such as apublic telephone switching network. User terminals can be fixed ormobile, such as a mobile telephone, and positioned near a gateway orremotely located.

A satellite can receive signals from and transmit signals to a userterminal provided the user terminal is within the “footprint” of thesatellite. The footprint of a satellite is the geographic region on thesurface of the earth covered by the satellite communication system. Insome satellite systems, a satellite's footprint is geographicallydivided into “beams,” through the use of beam forming antennas. Eachbeam covers a particular geographic region within a satellite'sfootprint.

Some satellite communications systems employ code division multipleaccess (CDMA) spread-spectrum signals, as disclosed in U.S. Pat. No.4,901,307, issued Feb. 13, 1990, entitled “Spread Spectrum MultipleAccess Communication System Using Satellite or Terrestrial Repeaters,”and U.S. Pat. No. 5,691,174, which issued Nov. 25, 1997, entitled“Method and Apparatus for Using Full Spectrum Transmitted Power in aSpread Spectrum Communication System for Tracking Individual RecipientPhase Time and Energy,” both of which are assigned to the assignee ofthe present invention, and are incorporated herein by reference.

In communication systems employing CDMA, separate communication linksare used to transmit communication signals to and from a gateway or basestation in a cellular system. A forward communication link refers tocommunication signals originating at the gateway or base station andtransmitted to a user terminal. A reverse communication link refers tocommunication signals originating at a user terminal and transmitted tothe gateway or base station. In situations where satellite diversity isdesired, the gateway establishes two or more forward links for a givenuser terminal, where each forward link is established on a beam from adifferent satellite. For example, in a two satellite diversityconfiguration a first forward link is established on a beam projected bya first satellite and a second forward link is established on a beamprojected by a second satellite. In the above example, the user terminalreceives information or data from the gateway on both the first andsecond beam. Satellite diversity provides increased system performancebecause fewer communication links or calls will likely be dropped. Forexample, if the beam carrying the first forward link is blocked by anobstruction (such as, a tall building), the connection between the userterminal and gateway will continue uninterrupted on the second forwardlink. The user will be unaware of the beam blockage. Consequently, beamsource diversity is commonly desired in a multi-beam communicationsystem.

In a satellite-based communications system where the satellites are notstationary with respect to a point on the surface of the earth, thegeographic area covered by a given satellite is constantly changing. Asa result, a user terminal that was at one time positioned within aparticular beam of a particular satellite can at a later time bepositioned within a different beam of the same satellite and/or within adifferent beam of a different satellite. Furthermore, because satellitecommunication is wireless, a user terminal is free to move about. Thus,even in systems where the satellites are stationary with respect to apoint on the surface of the earth, it is likely that over time a userterminal will be covered by different beams. Consequently, if acommunication link between a user terminal and a gateway is establishedon a first beam and the communication link is not established on otherbeams prior to the user terminal no longer being covered by the firstbeam, then, at some point, the user terminal will no longer be able tocommunicate with the gateway using the established communication link.As a result, an active call between the user terminal and the gatewaywill be dropped. Dropping calls in a communication system is a seriousproblem for service providers who strive to provide uninterruptedcommunication services. A similar call dropping problem may occur formobile users moving around in sectored cells in terrestrialcommunication systems. That is, where the cells are subdivided into twoor more smaller service areas which are covered at differing frequenciesor using different code spaces. Here, mobile users may travel along orrepeatedly cross sector boundaries within a cell, depending on suchfactors as cell and sector size and local physical environment.

What is, therefore, needed is a system and method for reducing calldropping rates in a multi-beam communication system. The system andmethod should maintain a desired level of beam source diversity tofurther enhance the reliability of the communication system.

SUMMARY OF THE INVENTION

In a multi-beam-communication system having a user terminal, acommunication station for transmitting information to and receivinginformation from the user terminal and a plurality of beam sources,where each beam source projects a plurality of beams, and where acommunication link between the user terminal and the communicationstation is established on one or more beams, the present inventionprovides a system and method for reducing call dropping rates.Furthermore, the system and method of the present invention maintain adesired level of beam source diversity.

The method according to the present invention relies on a messagingprotocol between the communication station and the user terminal. Basedon messages sent from the user terminal to the communication station,the communication station can determine the most desirable beam(s) onwhich to transmit information or data to the user terminal. The messagessent from the user terminal to the communication station contain valuesrepresenting beam strengths as measured at the user terminal. Thecommunication station uses these values to select the most desirablebeams that should be used as a communication link between thecommunication station and the user terminal. The beams that should beused are the beams that if used will decrease call dropping rates andprovide the desired level of beam source diversity.

The method according to one embodiment of the present invention includesthe steps of: (1) transmitting from the communication station to theuser terminal a Beam Mask Message (BMM) containing a plurality of beamidentifiers, where each of the beam identifiers identifies a beamcurrently available to the communication station; (2) periodicallymeasuring at the user terminal a strength of each beam identified in theBMM; (3) periodically transmitting from the user terminal to thecommunication station a Pilot Strength Measurement Message (PSMM)containing a plurality of beam strength values, where each beam strengthvalue is a function of the measured strength of one of the beamsidentified in the BMM; (4) based on the beam strength values in thePSMM, selecting at the communication station one or more beams thatshould be used as a communication link between the communication stationand the user terminal (i.e., the communication station selects a newactive beam set); (5) at the communication station, transmittinginformation on all of the beams in the new active beam set; (6)transmitting from the communication station to the user terminal aHandoff Direction Message (HDM) if the one or more beams selected instep (4) are not the same one or more beams that are in the currentactive beam set, where the current active beam set consists of the oneor more beams on which a communication link between the communicationstation and the user terminal is already established; and (7) receivingat the communication station a Handoff Completion Message (HCM)transmitted from the user terminal after the user terminal receivesinformation on each of the beams in the new active beam set.

Based on the HDM, the user terminal can determine the one or more beamsthat the communication station selected in step (4) that should be usedas a communication link between the communication station and the userterminal. In one embodiment, the HDM includes a beam identifiercorresponding to each beam selected by the communication station in step(4). In another embodiment, the HDM includes an add beam set and a dropbeam set. The add beam set includes a beam identifier for each beamwithin the new active beam set that is not in the current active beamset. The drop beam set includes a beam identifier for each beam in thecurrent active beam set that is not in the new active beam set.

According to one embodiment, the plurality of beam strength valuesincluded in the PSMM include a plurality of values corresponding to astrongest beam in each satellite identified in the BMM. In anotherembodiment, the beam strength values in the PSMM are adjusted beamstrength values.

In one embodiment, the step of selecting at the communication stationone or more beams that should be used as a communication link betweenthe communication station and the user terminal includes the steps of:(1) selecting the strongest beam in the PSMM; (2) determining thestrongest alternate beam in the PSMM, where an alternate beam is a beamprojected by a satellite other than the satellite that projects the beamselected in step (1); and (3) selecting the strongest alternate beam inthe PSMM if the strength of the strongest beam in the PSMM minus thestrength of the strongest alternate beam in the PSMM is less than athreshold amount.

In another embodiment, the step of selecting one or more beams on whichto establish a communication link includes the steps of: (1) selectingthe strongest beam in the PSMM; (2) determining the strongest alternatebeam in the PSMM; (3) selecting the strongest alternate beam in the PSMMif the strength of the strongest beam in the PSMM minus the strength ofthe strongest alternate beam in the PSMM is less than or equal to afirst threshold amount; (4) if the strength of the strongest beam in thePSMM minus the strength of the strongest alternate beam in the PSMM isgreater than the first threshold amount, determining the strongestalternate beam in the current active set, where an alternate beam in thecurrent active set is a beam in the current active set that is projectedby a satellite other than the satellite that projects the beam selectedin step (1); and (5) selecting the strongest alternate beam in thecurrent active set if the strength of the strongest beam in the PSMMminus the strength of the strongest alternate beam in the current activeset is less than or equal to a second threshold amount. In oneembodiment of the present invention the second threshold amount isgreater than the first threshold amount.

In another embodiment, the user terminal continually measures the beamstrength of each beam in the current active set. If the beam strength ofa beam in the current active set is less than the beam strength of thatbeam as reported in the previous PSMM by a predetermined amount andremains so over a specified interval of time, then the user terminalwill transmit a new PSMM to the communication station.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the left-mostdigit(s) of a reference number identifies the drawing in which thereference number first appears.

FIG. 1 illustrates an exemplary wireless communication systemconstructed and operating according to one embodiment of the presentinvention.

FIG. 2A illustrates an exemplary satellite footprint according to oneembodiment of the present invention.

FIG. 2B illustrates a perspective view of a signal beam pattern betweena base station of FIG. 1 and the surface of the Earth;

FIG. 2C illustrates an exemplary signal pattern for a base station inFIG. 1 with typical theoretical sector boundaries and variations;

FIGS. 3A and 3B illustrate the position of a satellite with respect to auser at a first and second point in time, respectively.

FIGS. 3C and 3D illustrate the position of the user in FIGS. 3A and 3Bwithin the satellite's footprint at the first and second points in time,respectively.

FIGS. 4A and 4B illustrate the position of a first satellite and asecond satellite with respect to a user at a first and second point intime, respectively.

FIGS. 4C and 4D illustrate the position of the user in FIGS. 4A and 4Bwithin the first and second's satellite footprint at the first andsecond points in time, respectively.

FIGS. 5A and 5B illustrate a beam handoff procedure according to apreferred embodiment of the present invention.

FIG. 6A illustrates an exemplary Beam Mask Message.

FIG. 6B illustrates example measured beam strength values.

FIG. 6C illustrates an exemplary Pilot Adjust Message.

FIG. 6D illustrates example adjusted beam strength values.

FIG. 6E illustrates an exemplary Pilot Strength Measurement Message(PSMM).

FIG. 7 illustrates an exemplary procedure used by a user terminal forestablishing the contents of a PSMM.

FIG. 8 illustrates a procedure used by a gateway for selecting beams fora new active set according to a first embodiment.

FIG. 9 illustrates a procedure used by a gateway for selecting beams fora new active set according to a second embodiment.

FIG. 10 illustrates an exemplary message flow between a gateway and userterminal.

FIG. 11 illustrates an exemplary user terminal transceiver.

FIG. 12 illustrates an exemplary control unit of a user terminal.

FIG. 13 illustrates exemplary components of a gateway used in performingthe beam handoff algorithm.

FIG. 14 illustrates an exemplary gateway selector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

I. Introduction

The present invention is suited for use in multi-beam communicationsystems. Such communication systems include communication systemsemploying Earth orbiting satellites or highly sectorized cells. However,it will be apparent to those skilled in the relevant art that theconcept of the present invention can be applied to a variety ofsatellite systems even when not utilized for communications purposes.The present invention can also be applied to cells using a variety ofcell sectorization schemes, again, even when not utilized for usercommunications.

A preferred embodiment of the invention is discussed in detail below.While specific steps, configurations and arrangements are discussed, itshould be understood that this is done for illustrative purposes only. Aperson skilled in the relevant art will recognize that other steps,configurations and arrangements can be used without departing from thespirit and scope of the present invention. The present invention couldfind use in a variety of wireless information and communication systems,including those intended for position determination, and satellite andterrestrial cellular telephone systems. A preferred application is inCDMA wireless spread spectrum communication systems for mobile orportable telephone service.

II. A Typical Communications System

An exemplary wireless communication system in which the presentinvention is found useful, is illustrated in FIG. 1. It is contemplatedthat this communication system uses CDMA type communication signals, butthis is not required by the present invention. In a portion of acommunication system 100 illustrated in FIG. 1, one base station 112,two satellites 116 and 118, and two associated gateways or hubs 120 and122 are shown for effecting communications with two remote userterminals 124, 126, and 128. Typically, the base stations andsatellites/gateways are components of separate communication systems,referred to as being terrestrial and satellite based, although, this isnot necessary. The total number of base stations, gateways, orsatellites in such systems depends on desired system capacity and otherfactors well understood in the art.

The terms base station and gateway are also sometimes usedinterchangeably, each being a fixed central communication station, withgateways being perceived in the art as highly specialized base stationsthat direct communications through satellite repeaters while basestations (also sometimes referred to as cell-sites) use terrestrialantennas to direct communications within surrounding geographicalregions. Gateways have more ‘housekeeping tasks,’ with associatedequipment, to maintain satellite communication links, and any centralcontrol centers also typically have more functions to perform wheninteracting with gateways and moving satellites. However, the presentinvention finds application in systems using either gateways or basestations as communication stations.

User terminals 124, 126, and 128 each include a wireless communicationdevice such as, but not limited to, a cellular telephone, a datatransceiver, or a paging or position determination receiver, and can behand-held, vehicle-mounted, or fixed as desired. Here, the userterminals are illustrated as hand-held, vehicle-mounted, and fixedtelephones 124, 126, and 128 respectively. User terminals are sometimesalso referred to as subscriber units, mobile stations, or simply as‘users’ or ‘mobiles’ in some communication systems, depending onpreference.

Generally, beams from a beam source (such as base station 112 orsatellites 116 and 118) cover different geographical areas in predefinedpatterns. Beams at different frequencies, also referred to as CDMAchannels or ‘sub-beams’, can be directed to overlap the same region. Itis also readily understood by those skilled in the art that beamcoverage or service areas for multiple satellites, or antenna patternsfor multiple base stations, might be designed to overlap completely orpartially in a given region depending on the communication system designand the type of service being offered, and whether space diversity isbeing achieved.

While only two satellites are shown for clarity, a variety ofmulti-satellite communication systems have been proposed with anexemplary system employing on the order of 48 or more satellites,traveling in eight different orbital planes in Low Earth Orbit (LEO) forservicing a large number of user terminals. However, those skilled inthe art will readily understand how the teachings of the presentinvention are applicable to a variety of satellite system and gatewayconfigurations. This includes other orbital distances andconstellations, for example, those using Geostationary satellites wherebeam-switching results mostly from user terminal motion. In addition, avariety of base station configurations can also be used.

FIG. 1 illustrates some possible signal paths for establishingcommunications between user terminals 124, 126, and 128 and base station112, or through satellites 116 and 118, with gateways 120 and 122. Thebase station-user terminal communication links are illustrated by lines130, 132, and 134. The satellite-user terminal communication linksbetween satellites 116 and 118, and user terminals 124 126, and 128 areillustrated by lines 138, 140, 142, and 144. The gateway-satellitecommunication links, between gateways 120 and 122, and satellites 116and 118, are illustrated by lines 146, 148, 150, and 152. Gateways 120and 122, and base station 112, may be used as part of a one-way ortwo-way communication system or simply to transfer messages/informationor data to user terminals 124, 126, and 128.

FIG. 2A illustrates an exemplary satellite beam pattern 202, also knownas a footprint. As shown in FIG. 2A, the exemplary satellite footprint202 includes sixteen beams. Each beam covers a specific geographicalarea, although there usually is some beam overlap. The satellitefootprint shown in FIG. 2 includes an inner beam (beam 1), middle beams(beams 2-7), and outer beams (beams 8-16). This beam pattern is aparticular predefined pattern used to reach users positioned withinouter portions of the footprint, where the signal strength is lower dueto natural “roll-off” effect created by the surface of the earth,without creating additional interference. The beams are illustrated ashaving non-overlapping geometric shapes for purposes of illustrationonly. However, those skilled in the art will readily appreciate thatother beam patterns and shapes may be used in various communicationsystem designs.

As shown in FIG. 2B, base stations or cell cites in such a communicationsystem (100), including base station 112, project beams or signalswithin a cell 220 covering a predetermined service area on the Earth'ssurface in accordance with signal strength and local terrain. Cell 220consists of one overall coverage area formed by a series of separatebeams or signals that create sectors 222, projected in a generally wedgeshaped patterns. Here, cell 220 is formed using a series of six sectors222, not all having the same area or size. However, a variety ofpatterns, sectors, and sector sizes can be used, as would be known toone skilled in the art. As discussed further below, a user may move froma position X in one sector 222 to a position Y in a neighboring sector222 along a path illustrated by line 224. This occurs as a result ofeither user terminal movement or changing sector coverage or acombination of both.

An exemplary sector pattern is illustrated in further detail in FIG. 2C.In FIG. 2C, a series of sectors S1-S6 are shown in a generally circularpattern or cell 220. This cell is illustrated as having irregular edgesas a result of how the signals are projected by transponders or antennasystems and the impact of local terrain or structures, as known in theart. As illustrated, the sectors need not be uniform in size, and mayeven have their respective coverage areas adjusted during operation ofthe communication system. The sector beams or signals also createoverlapping sector boundaries or regions of coverage between adjacentsectors, with beam energies generally being tailored at transmission, todecrease more rapidly near the edges or boundaries, to decreaseoverlapping signal coverage. The overlapping boundaries are shown usingsolid and dashed lines for adjacent sector boundaries. The adjacentsectors in this example each use different PN codes or code offsets in amanner similar to the satellite sub-beams. Those skilled in the art arefamiliar with these types of patterns and the frequency and PN codeassignments used to form such patterns.

FIGS. 3A-4D best illustrate the problem identified by the inventors thatthe present invention is designed to overcome. FIG. 3A illustrates therelative position of satellite 118 to user 302 at a first point in time,and FIG. 3B illustrates the relative position of satellite 118 to user302 at a second point in time. FIG. 3C is an overhead view of user 302and the satellite beam pattern at the first point in time, and FIG. 3Dis an overhead view of user 302 and the satellite beam pattern at thesecond point in time. As shown in FIGS. 3C and 3D, at the first point intime, user 302 is primarily covered by beam six of satellite 118, and atthe second point in time user 302 is primarily covered by beam three ofsatellite 118. At the first point in time, user terminal 124 detectsbeam six as having the strongest signal as compared to the other beams.At the second point in time, user terminal 124 detects beam three ashaving the strongest signal. Consequently, if the active callestablished on beam six is not “handed off” (transferred to beam three)by the second point in time, the call may get dropped.

FIGS. 4A-4D illustrate beam source diversity. FIG. 4A illustrates therelative positions of satellites 118 and 116 to user 302 at a firstpoint in time, and FIG. 4B illustrates the relative positions ofsatellites 118 and 116 to user 302 at a second point in time. FIG. 4C isan overhead view of user 302 and the satellite beam pattern at the firstpoint in time, and FIG. 4D is an overhead view of user 302 and thesatellite beam pattern at the second point in time. As shown in FIGS. 4Cand 4D, at the first point in time, user 302 is primarily covered bybeam one of satellite 118, and at the second point in time user 302 isprimarily covered by beam fifteen of satellite 116, and beam eleven ofsatellite 118.

The problem recognized by the inventors is that it is easy to determinethe most desirable beam(s) to establish a communication link on if youhave exact knowledge of where the user terminal is within a satellite'sfootprint. But the gateway, which chooses which beam(s) to establish acommunication link on, does not know where the user terminal ispositioned. Moreover, even if the user position is known, blockage byphysical objects such as trees, buildings, etc. may render the “bestbeam(s)” unusable. As a result, the inventors have designed a beamhandoff procedure for choosing the most desirable beam(s) for a userterminal to receive traffic on given that the position of the userterminal is not known and given the possibility of beam blockage.

The procedure is aimed at reducing hand off rates and call droppingrates while maintaining a desired level of beam source diversity. Theprocedure relies on a messaging protocol between the gateway and theuser terminal. Based on messages sent from the user terminal to thegateway, the gateway can determine the most desirable beam(s) fortransmitting information to the user terminal. The messages sent fromthe user terminal to the gateway contain values representing beamstrengths as measured at the user terminal.

III. Description of the Beam Handoff Procedure

The beam hand off procedure will be described with reference toflowchart 500 illustrated in FIGS. 5A and 5B. The procedure assumes thatat least one communication link between the user terminal and a gatewayinitially exists on a beam. That is, the gateway has selected a beam onwhich to transmit data or information to the user terminal.

The beam hand off procedure begins at step 504. In step 504, the gatewaytransmits a Beam Mask Message (BMM) to the user terminal over theestablished communication link(s). The BMM contains a list of beamidentifiers. Each beam identifier in the list identifies a beam overwhich the gateway can transmit data or information. In addition tosending a BMM to the user terminal, the gateway can send a Pilot AdjustMessage (PAM) to the user terminal. A PAM contains one or more pilotadjust values. The pilot adjust values are used to implement loadbalancing, and will be discussed in further detail with respect to step508.

The gateway performs step 504 periodically. For example, the gateway maysend an updated BMM every minute. The period of one minute was chosenbecause within approximately each minute one or more new beams becomeavailable to the gateway.

FIG. 6A illustrates an exemplary BMM 600. As shown in FIG. 6A, BMM 600consists of a list of beam identifiers 602-614. Beam identifiers 602-614each identify a satellite/beam pair. For example, the first beamidentifier 602 in BMM 600 identifies beam one from satellite one, andthe second beam identifier 604 identifies beam three from satellite one.

The user terminal measures the beam strength of each beam identified inthe most recent BMM received from the gateway (step 506). FIG. 6Billustrates exemplary measured beam strength values for the beamsidentified in BMM 600. In one embodiment, the user terminal measures abeam strength by measuring the amount of energy in a pilot signalassociated with the beam. Pilot signals are used by user terminals toobtain initial system synchronization and time, frequency, and phasetracking of other signals transmitted by the gateway. A single pilotsignal is typically transmitted by each gateway for each frequency used,referred to as a CDMA channel or sub-beam, and shared by all userterminals receiving signals from that gateway on that frequency. Pilotsignal strength can be measured using one of several known techniques.For example, one such technique is disclosed in U.S. patent applicationSer. No. 08/722,330, filed on Sep. 27, 1996, entitled “Method andApparatus for Adjacent Service Area Handoff in Communication Systems,”which is incorporated herein by reference.

After measuring the beam strengths, the user terminal can optionallyadjust one or more of the measured beam strength values using the one ormore pilot adjust values that can optionally be sent from the gateway ina PAM (step 508). The pilot adjust values are used to implement loadbalancing. The pilot adjust values compensate for the difference betweenthe beam strengths of the beams projected by a particular satellite. Forexample, there are situations where the outer beams are made strongerthan the inner and middle beams. Thus, without the pilot adjust values,the outer beams will be selected by the gateway for establishing acommunication link far more often than the other beams. This couldpresent a load balancing problem. Therefore, to balance the load evenlyamong the beams, the gateway sends PAMs to the user terminal to adjustthe value of the beam strength being used.

FIG. 6C illustrates an exemplary PAM. As shown in FIG. 6C, PAM 650contains one or more adjustment values corresponding to one or morebeams listed in BMM 600. For example, PAM 650 contains an adjust valuefor beam eleven of satellite one and an adjust value for beam sixteen ofsatellite two. A PAM can be transmitted by the gateway at any time. Inmost instances, the PAM is sent as part of the BMM. The user terminaladds the adjustment values to the appropriate measured beam strengthvalues. FIG. 6D illustrates the adjusted beam strength values for thebeams identified in BMM 600 based on PAM 650.

After step 508, the process progresses to step 510, where the userterminal transmits a Pilot Strength Measurement Message (PSMM) to thegateway. It should be noted that steps 506-510 are performedperiodically by the user terminal. Selecting the appropriate period isvaluable. If the user terminal measures or reports too often, the userterminal will still be within the same beam(s) and, thus, report thesame signal level. This wastes system overhead in the traffic channelswhere the reporting is done because the user terminal is transferringinformation that has not changed. In addition, user terminal and gatewayprocessing capacity is being unnecessarily consumed. On the other hand,if the user terminal reports at too large an interval or over too long aperiod, then the user terminal may miss a good beam that has passed by.

In one embodiment, selecting the period is accomplished by establishingthe parameters for a specific system and simulating the resulting beamsand motion. Therefore, based on a given satellite constellation (number)and ephemeris (motion and location), one can predict the motion andrates of change for beams. From this, one can arrive at a reasonableprediction of the appropriate period. Historical data taken when asystem is in use can also be used to adjust this period, as desired. Inone embodiment, the period is ten seconds. That is, every ten seconds,the user terminal transmits a PSMM to the gateway.

The PSMM transmitted from the user terminal to the gateway contains oneor more beam identifiers from BMM 600 and corresponding beam strengthvalues. The corresponding beam strength values can be the adjusted orunadjusted beam strength values. In one embodiment of the invention, thePSMM contains at most six beam identifiers and their corresponding beamstrength values. However, other numbers of beams can be used dependingon well known factors such as system complexity, processing power,storage capacity, etc. The contents of an exemplary PSMM 660 isillustrated in FIG. 6E.

The flowchart of FIG. 7 illustrates a preferred procedure employed atthe user terminal for selecting the one or more beams (the one or morebeam identifiers from BMM 600 and corresponding beam strength values) toinclude in the PSMM. The objective of procedure 700 is to achieve adesired level of satellite diversity. Thus, at least one beam from everysatellite identified in the BMM that is visible to the user terminal isadded to the PSMM. For example, if the BMM identifies three differentsatellites that are all visible to the user terminal, the PSMM willcontain at least three beam identifiers and at least three correspondingbeam strength values, where each one of the at least three identifiersidentifies a beam from a different one of the three satellites.

Procedure 700 begins at step 704. In step 704, the user terminal usesthe adjusted measured beam strengths to determine the “strongest” beamprojected by each satellite identified in the BMM. The “strongest” beamis the beam having the largest corresponding adjusted beam strengthvalue. For each beam determined in step 704, the user terminal includeseach beam's beam identifier and corresponding adjusted beam strengthvalue in the PSMM (step 706). In the next step, the user terminaldetermines if more values can be added to the PSMM (step 708). The userterminal determines this by subtracting the number of beams in the PSMMfrom the maximum allowable number of beams that can be added to thePSMM. In a preferred embodiment, six is the maximum allowable number ofbeams that can be included in the PSMM. If more beams can be added tothe PSMM control passes to step 710, otherwise the procedure ends. Insteps 710 and 712, the user terminal selects the strongest beam notalready added to the PSMM and adds that beam's beam identifier andcorresponding beam strength value to the PSMM. After step 712, controlpasses back to step 708. In another embodiment, the user terminal usesthe unadjusted beam strength values when performing procedure 700.Consequently, the PSMM can contain unadjusted or adjusted beam strengthvalues.

After receiving a PSMM from the user terminal, the gateway determines anew active beam set (step 512). The new active beam set is the set ofbeams that should be used as communication links between the gateway andthe user terminal. FIGS. 8 and 9 illustrate two procedures (800 and 900)that can be used by the gateway in performing step 512 (i.e.,determining the new active beam set). Procedure 800 is referred to asthe Single Threshold Scheme (STS), and procedure 900 is referred to asthe Dual Threshold Hysteresis Scheme (DTHS). The STS (procedure 800)will be described first and the DTHS (procedure 900) will be describedsecond.

Procedure 800 begins at step 804. In step 804, the gateway selects thestrongest beam in the PSMM and adds that beam to the new active beamset. That is, the gateway selects the largest beam strength value fromthe PSMM, determines the beam corresponding to the selected value, andadds that beam to the new active beam set. Prior to step 804, the newactive beam set is set to “no beam.” That is, the new active beam set isinitialized and does not contain any beams.

In step 806, the gateway determines the strongest “alternate” beam inthe PSMM, if there is one. An “alternate” beam is any satellite beamwithin the PSMM that is projected by a satellite other than a satellitethat projects a beam that is in the new active set. The strongest“alternate” beam in the PSMM is, therefore, the alternate beam that hasthe largest beam strength value relative to the other alternate beams.To determine the strongest alternate beam within the PSMM, the gatewayfirst selects a subset of values from the PSMM, where the subset ofvalues includes all values in the PSMM that correspond to a beamprojected by a satellite other than a satellite that projects a beamthat is included in the new active set. Second, the gateway selects thelargest value from the subset. Third, the gateway determines the beamthat corresponds to the value selected in the previous step.

If a strongest alternate beam exists step 808 is performed, otherwisethe process ends. In step 808, the gateway compares the strength of thestrongest beam (SSB) in the PSMM (i.e., the beam selected in step 804)to the strength of the strongest alternate beam (SSAB) (i.e., the beamselected in step 806). If SSB minus SSAB is less than or equal to afirst threshold (T_(—)1), then the gateway adds the strongest alternatebeam in the PSMM to the new active beam set (step 810), otherwise theprocedure ends and the new active beam set will contain only thestrongest beam in the PSMM. In a preferred embodiment, T_(—)1 is on theorder of 4 dB. But other embodiments are anticipated, such as T_(—)1being 0 dB or T_(—)1 being infinitely large, in which case the strongestalternate beam will always be added to the new active set regardless ofits strength.

After step 810, the procedure continues to step 812. In step 812, thegateway determines whether or not additional alternate beams should beadded to the new active beam set. The number of alternative beams addedto the new active beam set is determined by the level of desiredsatellite diversity. For example, if it is desirable to have only a twosatellite diversity configuration, then the gateway will only attempt toadd one alternative beam to the new active beam set. However, if an Nsatellite diversity configuration is desired, then the gateway willattempt to add N−1 alternative beams to the new active beam set.

The DTHS is similar to the STS. For example, the first four steps ofprocedure 900 are the same as the first four steps of procedure 800. Thedifference between procedure 800 and procedure 900 is that in procedure900 step 902 is performed if SSB minus SSAB is not less than or equal toT_(—)1, whereas in procedure 800, if SSB minus SSAB is not less than orequal to T_(—)1, the procedure ends.

In step 902 the gateway selects the strongest alternate beam in thecurrent active set, if there is one. The current active set refers tothe set of active beams, where an active beam is a beam over which acommunication link is already established between the gateway and theuser terminal. An alternate beam in the current active set is a beam inthe current active set that is projected by a satellite other than asatellite that projects a beam that is in the new active beam set. Todetermine the strongest alternate beam within the current active set,the gateway first selects a subset of values from the PSMM, where thesubset of values includes all values in the PSMM set that correspond toa beam in the current active set that is projected by a satellite otherthan the satellite(s) that project(s) the beam(s) that are in the newactive beam set. Second, the gateway selects the largest value from thesubset. Third, the gateway determines the beam that corresponds to thevalue selected in the previous step.

If step 902 is successful, then step 904 is performed, otherwise theprocess ends. In step 904, the gateway determines whether or not thedifference between the strength of the strongest beam (SSB) in the PSMMand the strength of the strongest alternate satellite beam in thecurrent active set (SSASB_CAS) is less than or equal to a secondthreshold (T_(—)2). If the difference is less than or equal to T_(—)2,the strongest alternate beam in the current active set is added to thenew active set (step 906), otherwise the process ends.

After step 906, the procedure continues to step 908. In step 908, thegateway determines whether or not additional alternate beams should beadded to the new active beam set. The number of alternate beams added tothe new active beam set is determined by the level of desired satellitediversity. For example, if it is desirable to have only a two satellitediversity configuration, then the gateway will only attempt to add onealternative beam to the new active beam set. However, if an N satellitediversity configuration is desired, then the gateway will attempt to addN−1 alternative beams to the new active beam set.

Preferably, T_(—)2 is greater than T_(—)1, and T_(—)2 is 6 dB whenT_(—)1 is 4 dB. However, other values can be used for these thresholds.In the situation where T_(—)2 is greater than T_(—)1, the gateway givespreference to beams in the current active beam set, thereby reducinghandoffs due to temporary beam signal strength fluctuations caused by,among other things, specular reflection. Thresholds T_(—)1 and T_(—)2are chosen in part based on known satellite orbital distances (heightabove Earth) and velocities, which together determines the angles andrate of change of specular reflection.

The advantage of the Single Threshold Scheme (STS) is its implementationsimplicity as compared to the Dual Threshold Hysteresis Scheme (DTHS).The DTHS, however, has a lower handoff rate than the STS. The DTHSachieves a lower handoff rate by smoothing out the chattering effectcaused by beam signal strength fluctuations due to specular reflections.The chattering effect is a situation where the gateway alternately addsand drops a particular beam over a short time interval. At thetermination of either process 800 or 900, the new active beam set willcontain the beams that should be used as a communication link connectingthe gateway with the user terminal.

After step 512, step 514 is performed. In step 514, the gatewaydetermines if the new active beam set is equivalent to the currentactive beam set. The current active beam set consists of all the beamson which a communication link between the gateway and the user terminalis already established. If the new active beam set is the same as thecurrent active beam set, the gateway does not initiate handoff, therebyallowing the user terminal to continue using the beams in the currentactive beam set (step 515). If the new active beam set is not equal tothe current active beam set, the gateway will initiate beam handoff(steps 516-530).

In a preferred embodiment, the beam handoff is a “soft” beam handoff.That is, the gateway will not break the connections in the currentactive beam set until it receives confirmation from the user terminalthat the user terminal is successfully receiving information on thebeam(s) in the new active beam set. Consequently, as the first step ofinitiating a soft handoff, the gateway starts transmitting traffic onthe beams in the new active beam set that are not in the current activebeam set if there are any such beams (step 516). In the next step, thegateway sends a handoff direction message (HDM) to the user terminal(step 518). In one embodiment, the HDM can contain two sets of beamidentifiers, and add beam set and a drop beam set. The add beam setcontains a beam identifier for each beam within the new active beam setthat is not in the current active beam set. It is possible for the addbeam set to be empty, in which case the HDM will only contain the dropbeam set. The drop beam set contains a beam identifier for each beam inthe current active beam set that is not in the new active beam set. Likethe add beam set, it is possible for the drop beam set to be empty, inwhich case the HDM will only contain the add beam set. In a secondembodiment, the HDM contains a beam identifier corresponding to eachbeam in the new active beam set. Upon receiving the HDM according to thesecond embodiment, the user terminal can determine the add beam set andthe drop beam set because the user terminal has knowledge of which beamsare in the current active set.

In either the first or second embodiments, the user terminal, uponreceiving an HDM, begins to receive traffic over the beams identified inthe add beam set (step 520). Once the user terminal begins receivinginformation on the beams identified in the add beam set, the userterminal will stop receiving information on the beams identified in thedrop beam set (step 522). The user terminal then transmits a handoffcompletion message (HCM) to the gateway (step 524). Upon receiving theHCM from the user terminal, the gateway stops transmitting traffic onthe beams identified in the drop beam set (step 526). In this manner,soft beam handoff is accomplished.

An example of a flow of messages between the gateway and the userterminal is illustrated in FIG. 10. As shown in FIG. 10, the handoffprocess begins with the gateway periodically (e.g., every 60 seconds)sending a BMM/PAM to the user terminal. Upon receiving a BMM the userterminal periodically (e.g., every 10 seconds) sends a PSMM to thegateway. Upon receiving a PSMM from the user terminal, the gatewaydetermines the most desirable beams to use (i.e., determines a newactive beam set). If the new active beam set equals the current activebeam set, then the gateway will not initiate a beam handoff. But, if thenew active beam set is different than the current active beam set, thegateway will send an HDM to the user terminal. The user terminal willrespond with an HCM.

As shown in FIG. 10, the user terminal typically sends a PSMM only aftera predetermined amount of time has elapsed since a previous PSMM hasbeen sent. But there is at least one situation where it is recommendedfor the user terminal to send to the gateway an “unscheduled” PSMM. Anunscheduled PSMM is a PSMM that is sent to the gateway whenever asatellite blockage occurs, regardless of when the previous PSMM wassent. Satellite blockage is defined as the condition where the presentstrength of the active beam is less than the strength of the active beamas reported in the most recently transmitted PSMM minus a thresholdamount (T_Loss) and remains so over a specified interval of time(T_TLoss). When this situation occurs, the user terminal will performsteps 506-510, thereby transmitting an unscheduled PSMM. As an example,assume the present strength of an active beam is 7 or less over aninterval of T_TLoss and assume that the strength of the active beam asreported in the previous PSMM is 10. If 7<(10-T_Loss), then the userterminal will perform an unscheduled PSMM. The gateway will then performstep 512 as above. That is, the gateway uses the PSMM to determine whichbeam(s) should be used for transmitting traffic to the user terminal.

IV. User Terminal Transceiver Description

An exemplary transceiver 1100 for use in a user terminal 124 isillustrated in FIG. 11. Such transceivers are known in the art anddiscussed in patents such as U.S. Pat. No. 5,109,390, entitled“Diversity Receiver In A CDMA Cellular Telephone System,” which isincorporated herein by reference. Transceiver 1100 uses at least oneantenna 1110 for receiving communication signals which are transferredto an analog receiver 1114, where they are down converted, amplified,and digitized. A duplexer element 1112 is typically used to allow thesame antenna to serve both transmit and receive functions. However, somesystems employ separate antennas for operating at different transmit andreceive frequencies.

The digital communication signals output by analog receiver 1114 aretransferred to at least one digital data receiver 1116A and preferablyat least one searcher receiver 1118. Additional digital data receivers1116B-1116N can be used to obtain desired levels of signal diversity orreceive multiple signals, depending on the acceptable level oftransceiver 1100 complexity, as would be apparent to one skilled in therelevant art. Additional searcher receivers can be used to implementmore complex signal searching techniques.

At least one user terminal control unit 1120 is coupled to digital datareceivers 1116A-1116N and searcher receiver 1118. Control unit 1120provides, among other functions, basic signal processing, timing, powerand beam handoff control or coordination, and selection of frequencyused for signal carriers. Another basic control function often performedby control unit 1120 is the selection or manipulation of PN codesequences or orthogonal functions to be used for processingcommunication signal waveforms. Control unit 1120 signal processing caninclude a determination of relative signal strength and computation ofvarious related signal parameters. Such computations of signalparameters, such as timing and frequency may include the use ofadditional or separate dedicated circuitry to provide increasedefficiency or speed in measurements or improved allocation of controlprocessing resources. For example, a signal strength measuring elementcan be connected to the analog receiver for using certain informationavailable to determine the signal strength or power for the overallreceived analog signal. This measuring element can also be connected toreceive outputs of, or data available from, the digital data andsearcher receivers for measuring the energy or power in specific signalsbeing received or demodulated.

The outputs of digital data receivers 1116A-1116N are coupled to digitalbaseband circuitry 1122 within the user terminal. User digital basebandcircuitry 1122 comprises processing and presentation elements used totransfer information to and from a user terminal user. That is, signalor data storage elements, such as transient or long term digital memory;input and output devices such as display screens, speakers, keypadterminals, and handsets; A/D elements, vocoders and other voice andanalog signal processing elements; etc., all form parts of thesubscriber baseband circuitry using elements well known in the art. Ifdiversity signal processing is employed, user digital baseband circuitry1122 can comprise one or more diversity combiners and decoders. Some ofthese elements may also operate under the control of, or incommunication with, control unit 1120.

When voice or other data is prepared as an output message orcommunications signal originating with the user terminal, user digitalbaseband circuitry 1122 is used to receive, store, process, andotherwise prepare the desired data for transmission. User digitalbaseband circuitry 1122 provides this data to a transmit modulator 1126operating under the control of control unit 1120. The output of transmitmodulator 1126 is transferred to a power controller 1128 which providesoutput power control to a transmit power amplifier 1130 for finaltransmission of the output signal from antenna 1110 to a gateway, orbase station.

User terminal 1100 can also employ a precorrection element 1132 in thetransmission path to adjust the frequency of the outgoing signal. Thiscan be accomplished using well known techniques of up- ordown-conversion of the transmission waveform. User terminal 1100 canalso employ a precorrection element 1132 in the transmission path toadjust the timing of the outgoing signal. This can be accomplished usingwell known techniques of adding or subtracting delay in the transmissionwaveform.

Information or data corresponding to one or more measured signalparameters for received communication signals, or one or more sharedresource signals, can be sent to the gateway using a variety oftechniques known in the art. For example, the information can betransferred as a separate information signal or be appended to othermessages prepared by user digital baseband circuitry 1122.Alternatively, the information can be inserted as predetermined controlbits by transmit modulator 1126 or transmit power controller 1128 undercontrol of control unit 1120 using known techniques.

Digital receivers 1116A-N and searcher receiver 1118 are configured withsignal correlation elements to demodulate or track specific signals.Searcher receiver 1118 is used to search for pilot signals, or otherrelatively fixed pattern strong signals. The pilot channel is simply asignal that is not modulated by data, and may use a constant-value(pattern) or tone-type input, effectively transmitting only PN spreadingcodes. The digital receivers 1116A-N are used to demodulate othersignals associated with detected pilot signals. For purposes ofdetermining signal strength, however, a data receiver can be assigned toprocess the pilot signal after acquisition to accurately determine theratio of signal chip energies to signal noise. Generally, pilot signalchip energies are integrated over predetermined intervals, such assymbol periods, to formulate pilot signal strength. Therefore, theoutputs of receivers 1116A-N can be monitored to determine the energyin, or frequency of, the pilot signal or other signals. These receiversalso employ frequency tracking elements that can be monitored to providecurrent frequency and timing information to control processor or unit1120 for signals being demodulated

As stated above, control unit 1120 provides, among other functions, beamhandoff control. That is, for example, control unit 1120 receives BMMsfrom a gateway, measures beam strengths by monitoring the energy of thepilot signals, and transmits PSMMs to a gateway. An example control unit1120 is shown in FIG. 12. The control unit 1120 includes one or moreprocessors, such as processor 1204. The processor 1204 is connected to acommunication bus 1202.

Control unit 1120 may be implemented in a software-controlled processorprogrammed to perform the functions described herein. That is,implemented as well known standard elements or generalized function orgeneral purpose hardware including a variety of digital signalprocessors, programmable electronic devices, or computers that operateunder the control of special function software or firmware programmingto perform the desired functions.

Control unit 1120 also includes a main memory 1206, preferably randomaccess memory (RAM), and can also include a secondary memory 1208. Thesecondary memory 1208 can include, for example, means for allowingcomputer programs or other instructions to be loaded into control unit1120. Such means can include, for example, a storage device 1222 and aninterface 1220. Examples of such can include a memory chip (such as anEPROM, or PROM) and associated socket, and other storage devices 1222and interfaces 1220 which allow software and data to be transferred fromthe storage device 1222 to control unit 1120.

Control unit 1120 can also include a communications interface 1224.Communications interface 1224 allows software and data to be transferredbetween control unit 1120 and digital data receiver 1116, for example.

In this document, the terms “computer program medium” and “computerusable medium” are used to generally refer to media such as removablestorage 1222 and main memory 1206. These computer program products aremeans for providing software to control unit 1120.

Control or computer programs (also called computer control logic) arestored in main memory and/or secondary memory 1208. Such computerprograms, when executed, enable the control unit 1120 to perform thefeatures of the present invention as discussed herein. In particular,for example, the computer programs, when executed, enable the processor1204 to perform measured beam strength value comparisons. Accordingly,such computer programs represent controllers of the control unit 1120.

In another embodiment, the control unit 1120 is implemented primarily inspecialized hardware configured for this function using, for example,hardware components such as application specific integrated circuits(ASICs), or one or more circuit card assemblies. Implementation of thehardware state machine so as to perform the functions described hereinwill be apparent to persons skilled in the relevant art(s).

In yet another embodiment, control unit 1120 is implemented using acombination of both hardware and software.

V. Gateway

FIG. 13 illustrates components of gateway 120 that enable the gateway toperform the features of the present invention. As shown in FIG. 13,gateway 120 includes a gateway switching subsystem (GSS) 1301 connectedto the public switched telephone network (PSTN) 1390, a selector banksubsystem (SBS) 1302, a time and frequency unit (TFU) 1318, a gatewaycontroller (GC) 1320, a CDMA interconnect subsystem (CIS) 1322, agateway transmission system (GTS) 1304, and a gateway RF subsystem (GRS)1310. The GTS includes a forward link transmission system (FLGTS) 1306and a reverse link transmission system (RLGTS) 1308. FLGTS 1306 takespacketized data from SBS 1302, modulates and frequency converts the datato an IF frequency (800-1000 MHz), and delivers it to the gateway RFsubsystem (GRS) 1310, which delivers it to antenna 1312 for transmissionto a satellite. The satellite then relays the signal to a user terminal.The packetized data received at FLGTS 1306 from SBS 1302 includes:traffic frames; overhead message frames; and power control information.The traffic frames may contain BMMs, PAMs, and HDMs. In this manner, agateway transmits BMMs, PAMs, and HDMs to a user terminal.

RLGTS 1308 receives IF signals from GRS 1310, down converts anddemodulates them, and sends packetized data to SBS 1302 for furtherprocessing. The packetized data received at SBS 1302 includes: trafficframes and overhead message frames transmitted from a user terminal.PSMMs and HCMs are transmitted from a user terminal to a gateway in atraffic frame. In this manner, PSMMs and HCMs are received at thegateway.

SBS 1302 includes one or more selectors 1314 for processing voice callsand performing the actions necessary to accomplish beam handoff. Forexample, selectors 1314 evaluate PSMMs sent from a user terminal todetermine which, if any, new beams are to be added, and which, if any,are to be dropped. Before adding beams, the SBS 1302 sends a forwardlink resource request to the GC 1320. If the resource request isgranted, a selector 1314 signals FLGTS 1306 to begin transmittingforward traffic on the new beam. Once FLGTS 1306 starts transmitting thetraffic, the selector 1314 sends an HDM to the user terminal. Uponreceiving the traffic on the new beam, the user terminal send an HCM tothe selector 1314. After receiving the HCM, the selector 1314 signalsFLGTS 1306 to stop transmitting traffic on a dropped beam, if there isone.

As in the case of control unit 1120, selector 1314 may be implemented ina software-controlled processor programmed to perform the functionsdescribed herein. That is, implemented as well known standard elementsor generalized function or general purpose hardware including a varietyof digital signal processors, programmable electronic devices, orcomputers that operate under the control of special function software orfirmware programming to perform the desired functions.

An example selector 1314 is shown in FIG. 14. The selector 1314 includesone or more processors, such as processor 1404. The processor 1404 isconnected to a communication bus 1402. Selector 1314 also includes amain memory 1406, preferably random access memory (RAM), and can alsoinclude a secondary memory 1408. The secondary memory 1408 can include,for example, means for allowing computer programs or other instructionsto be loaded into selector 1314. Such means can include, for example, aremovable storage unit 1422 and an interface 1420. Examples of such caninclude a removable memory chip (such as an EPROM, or PROM) andassociated socket, hard drives, magnetic tape, compact disc and othersimilar optical storage devices, and other removable storage units 1422and interfaces 1420 which allow software and data to be transferred fromthe removable storage unit 1422 to selector 1314. Selector 1314 can alsoinclude a communications interface 1424. Communications interface 1424allows data to be transferred between selector 1314 and FLGTS, forexample.

Computer programs (also called computer control logic) are stored inmain memory and/or secondary memory 1408. Such computer programs, whenexecuted, enable the selector 1314 to perform the features of thepresent invention as discussed herein. In particular, for example, thecomputer programs, when executed, enable the processor 1404 to performmeasured beam strength value comparisons. Accordingly, such computerprograms represent controllers of the selector 1314.

In another embodiment, the selector 1314 is implemented primarily inhardware configured for this function using, for example, hardwarecomponents such as application specific integrated circuits (ASICs), orone or more circuit card assemblies. Implementation of the hardwarestate machine so as to perform the functions described herein will beapparent to persons skilled in the relevant art(s).

In yet another embodiment, selector 1314 is implemented using acombination of both hardware and software.

VI. Conclusion

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. While the invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. In a multi-beam communication system having auser terminal, a communication station, and a beam source, wherein thebeam source projects a plurality of beams, and wherein a communicationlink between the user terminal and communication station is establishedon at least one of the plurality of beams, a method for reducing calldropping rates, comprising the steps of: (1) receiving at the userterminal a plurality of beam identifiers transmitted from thecommunication station; (2) measuring at the user terminal a beamstrength of each beam identified by said plurality of beam identifiers;(3) transmitting from the user terminal to the communication stationaplurality of beam strength values, wherein each of said plurality ofbeam strength values is a function of a measured beam strength of a beamidentified by one of said plurality of beam identifiers; and (4)receiving at the user terminal a handoff direction message transmittedby the communication station, wherein: (a) based on said handoffdirection message, the user terminal determines which beam or beamsshould be used for receiving information transmitted from thecommunication station, (b) said handoff direction message comprises abeam identifier corresponding to each beam in a new active beam set, and(c) said new active beam set includes beams selected by thecommunication station that should be used as a communication linkbetween the communication station and the user terminal.
 2. The methodof claim 1, wherein upon receiving said handoff direction message, theuser terminal begins receiving traffic over each beam in said new activebeam set that is not in a current active beam set, wherein said currentactive beam set includes all beams on which a communication link betweenthe communication station and the user terminal is already established.3. The method of claim 2, further comprising the step of transmitting ahandoff completion message from the user terminal to the communicationstation after the user terminal begins receiving traffic over each beamin said new active beam set that is not in said current active beam set.4. In a multi-beam communication system having a user terminal, acommunication station, and a beam source, wherein the beam sourceprojects a plurality of beams, and wherein a communication link betweenthe user terminal and communication station is established on at leastone of the plurality of beams, a method for reducing call droppingrates, comprising the steps of: (1) receiving at the user terminal aplurality of beam identifiers transmitted from the communicationstation; (2) measuring at the user terminal a beam strength of each beamidentified by said plurality of beam identifiers; (3) transmitting fromthe user terminal to the communication station a plurality of beamstrength values, wherein each of said plurality of beam strength valuesis a function of a measured beam strength of a beam identified by one ofsaid plurality of beam identifiers; and (4) receiving at the userterminal a handoff direction message transmitted by the communicationstation, wherein, (a) based on said handoff direction message, the userterminal determines which beam or beams should be used for receivinginformation transmitted from the communication station, (b) said handoffdirection message comprises a beam identifier corresponding to each beamin a new active beam set that is not in a current active beam set, (c)said current active beam set includes all beams on which a communicationlink between the communication station and the user terminal is alreadyestablished, and (d) said new active beam set includes beams selected bythe communication station that should be used as a communication linlbetween the communication station and the user terminal.
 5. The methodof claim 4, wherein upon receiving said handoff direction message, theuser terminal begins to receive traffic on a beam identified by a beamidentifier in said handoff direction message.
 6. Th e method of claim 5,further comprising the step of transmitting a handoff completion messagefrom the user terminal to the communication station.
 7. In a multi-beamcommunication system having a user terminal, a communication station,and a beam source, wherein the beam source projects a plurality ofbeams, and wherein a communication link between the user terminal andcommunication station is established on at least one of the plurality ofbeams, a method for reducing call dropping rates, comprising the stepsof: (1) receiving at the user terminal a plurality of beam identifierstransmitted from the communication station; (2) measuring at the userterminal a beam strength of each beam identified by said plurality ofbeam identifiers; (3) transmitting from the user terminal to thecommunication station a plurality of beam strength values, wherein eachof said plurality of beam strength values is a function of a measuredbeam strength of a beam identified by one of said plurality of beamidentifiers; and (4) receiving at the user terminal a handoff directionmessage transmitted by the communication station, wherein, based on saidhandoff direction message, the user terminal determines which beam orbeams should be used for receiving information transmitted from thecommunication station, wherein (a) said handoff direction messagecomprises a beam identifier corresponding to each beam in a currentactive beam set that is not in a new active beam set, (b) said newactive beam set includes beams selected by the communication stationthat should be used as a communication link between the communicationstation and the user terminal, and (c) said current active beam setincludes all beams on which a communication link between thecommunication station and the user terminal is already established. 8.The method of claim 7, wherein, after receiving said handoff directionmessage, the user terminal stops receiving traffic on a beam identifiedby a beam identifier in said hand off direction message.
 9. In amulti-beam communication system having a user terminal, a communicationstation, and a plurality of beam sources, wherein each beam sourceprojects a plurality of beams, and wherein a communication link betweenthe user terminal and the communication station is established on one ormore beams, a method for reducing call dropping rates, comprising thesteps of: (1) transmitting from the communication station to the userterminal a plurality of beam identifiers, wherein said plurality of beamidentifiers identifies a plurality of beams and a plurality of beamsources currently in use by the communication station; (2) receiving atthe communication station a plurality of beam strength valuestransmitted by the user terminal, wherein each of said plurality of beamstrength values is a function of a measured beam strength of a beamidentified by one of said plurality of beam identifiers; (3) determiningat the communication station a new active beam set based on saidplurality of beam strength values, wherein said new active beam setincludes one or more beams that should be used as a communication linkbetween the communication station and the user terminal, including thesteps of: (a) selecting a first beam having the strongest beam strengthbased on said plurality of beam strength values, wherein said first beamis projected by a first beam source, (b) including said first beam insaid new active beam set, (c) selecting a first subset of beam strengthvalues from said plurality of beam strength values, wherein said firstsubset of beam strength values includes each of said plurality of beamstrength values that correspond to a beam projected by a beam sourceother than said first beam source, (d) selecting a second beam havingthe strongest beam strength based on said first subset of beam strengthvalues, and (e) including said second beam in said new active beam setif the strength of said first beam minus the strength of said secondbeam is less than or equal to a threshold amount greater than or equalto zero; and (4) transmitting traffic from the communication station tothe user terminal on said beams within said new active beam set.
 10. Themethod of claim 9, wherein the step of determining at the communicationstation a new active beam set further comprises the steps of: (6)selecting a second subset of beam strength values from said firstsubset, wherein each beam strength value within said second subset ofbeam strength values corresponds to an active beam, wherein an activebeam is a beam over which information is currently being transmittedbetween the user terminal and the communication station; (7) selecting athird beam having the strongest beam strength based on said secondsubset of beam strength values; and (8) including said third beam insaid new active beam set if the strength of said first beam minus thestrength of said third beam is less than or equal to a second thresholdamount.
 11. The method of claim 10, wherein said second threshold amountis greater than or equal to said first threshold amount.
 12. In amulti-beam communication system having a user terminal, a communicationstation, and a beam source, wherein the beam source projects a pluralityof beams, and wherein a communication link between the user terminal andcommunication station is established on at least one beam, a systemwithin the user terminal for reducing call dropping rates, comprising:beam identifier receiving means for receiving a plurality of beamidentifiers transmitted from the communication station, wherein saidbeam identifiers identify a plurality of beams currently in use by thecommunication station; beam strength measuring means for measuring abeam strength of each beam identified by said plurality of beamidentifiers; transmitting means for transmitting to the communicationstation a plurality of beam strength values, wherein each of saidplurality of beam strength values is a function of a measured beamstrength of a beam identified by one of said plurality of beamidentifiers; handoff direction message receiving means for receiving ahandoff direction message transmitted by the communication station; andmeans for determining which beam or beams should be used for receivinginformation transmitted from the communication station based on saidhandoff direction message; wherein said handoff direction messagecomprises a beam identifier corresponding to each beam in a currentactive beam set that is not in a new active beam set, said new activebeam set includes beams selected by the communication station thatshould be used as a communication link between the communication stationand the user terminal, and said current active beam set includes allbeams on which a communication link between the communication stationand the user terminal is already established.
 13. The system of claim12, wherein, after receiving said handoff direction message, the userterminal stops receiving traffic on a beam identified by a beamidentifier in said hand off direction message.
 14. In a multi-beamcommunication system having a user terminal, a communication station,and a beam source, wherein the beam source projects a plurality ofbeams, and wherein a communication link between the user terminal andcommunication station is established on at least one beam, a systemwithin the user terminal for reducing call dropping rates, comprising:means for receiving a plurality of beam identifiers transmitted from thecommunication station, wherein said beam identifiers identify aplurality of beams currently in use by the communication station; meansfor measuring a beam strength of each beam identified by said pluralityof beam identifiers; transmitting means for transmitting to thecommunication station a plurality of beam strength values, wherein eachof said plurality of beam strength values is a function of a measuredbeam strength of a beam identified by one of said plurality of beamidentifiers; means for receiving beam strength adjustment valuestransmitted by the communication station; and adjusting means foradjusting said plurality of beam strength values according to saidreceived beam strength adjustment values, wherein said adjusting meansadjusts said plurality of beam strength values according to saidreceived beam strength adjustment values prior to said transmittingmeans transmitting said plurality of beam strength values to thecommunication station, whereby the communication station will receiveadjusted beam strength values.
 15. In a multi-beam communication systemhaving a user terminal, a communication station, and a plurality of beamsources, wherein each beam source projects a plurality of beams, andwherein a communication link between the user terminal and communicationstation is established on one or more beams, a system within thecommunication station for reducing call dropping rates, comprising:means for identifying a plurality of beams currently in use by thecommunication station; means for transmitting to the user terminal aplurality of beam identifiers, wherein each beam identifier identifiesone of said beams currently in use by the communication station; meansfor receiving a plurality of beam strength values transmitted by theuser terminal, wherein each of said plurality of beam strength values isa function of a measured beam strength of a beam identified by one ofsaid plurality of beam identifiers; means for determining a new activebeam set based on said plurality of beam strength values, wherein saidnew active beam set includes one or more beams that should be used as acommunication link between the communication station and the userterminal, including: means for selecting a first beam having thestrongest beam strength based on said plurality of beam strength values,wherein said first beam is projected by a first beam source, means forincluding said first beam in said new active beam set, means forselecting a first subset of beam strength values from said plurality ofbeam strength values, wherein said first subset of beam strength valuesincludes each of said plurality of beam strength values that correspondto a beam projected by a beam source other than said first beam source,means for selecting a second beam having the strongest beam strengthbased on said first subset of beam strength values, and means forincluding said second beam in said new active beam set if the strengthof said first beam minus the strength of said second beam is less thanor equal to a threshold amount greater than or equal to zero; and meansfor transmitting traffic to the user terminal on said beams within saidnew active beam set.
 16. The system of claim 15, wherein said means fordetermining a new active beam set further comprises: (f) means forselecting a second subset of beam strength values from said firstsubset, wherein each beam strength value within said second subset ofbeam strength values corresponds to an active beam, wherein an activebeam is a beam over which information is currently being transmittedbetween the user terminal and the communication station; (g) means forselecting a third beam having the strongest beam strength based on saidsecond subset of beam strength values; and (h) means for including saidthird beam in said new active beam set, wherein said means for includingsaid third beam in said new active beam set includes said third beam insaid new active beam set if the strength of said first beam minus thestrength of said third beam is less than or equal to a second thresholdamount.
 17. The system of claim 16, wherein said second threshold amountis greater than or equal to said first threshold amount.
 18. For use ina multi-beam communication system having a user terminal, acommunication station, and a plurality of beam sources, wherein eachbeam source projects a plurality of beams, and wherein a communicationlink between the user terminal and communication station is establishedon one or more beams, computer program logic stored on a computeruseable medium, comprising: means for enabling the communication stationto identify a plurality of beams currently in use by the communicationstation; means for enabling the communication station to transmit to theuser terminal a plurality of beam identifiers, wherein each beamidentifier identifies one of said beams currently in use by thecommunication station; means for enabling the communication station toreceive a plurality of beam strength values transmitted from the userterminal, wherein each of said plurality of beam strength values is afunction of a measured beam strength of a beam identified by one of saidplurality of beam identifiers; means for enabling the communicationstation to determine, based on said plurality of beam strength values, anew active beam set, wherein said new active beam set includes one ormore beams that should be used as a communication link between thecommunication station and the user terminal, including: means forenabling the communication station to select a first beam having thestrongest beam strength based on said plurality of beam strength values,wherein said first beam is projected by a first beam source, means forenabling the communication station to include said first beam in saidnew active beam set, means for enabling the communication station toselect a first subset of beam strength values from said plurality ofbeam strength values, wherein said first subset of beam strength valuesincludes each of said plurality of beam strength values that correspondto a beam projected by a beam source other than said first beam source,means for enabling the communication station to select a second beamhaving the strongest beam strength based on said first subset of beamstrength values, and means for enabling the communication station toinclude said second beam in said new active beam set if the strength ofsaid first beam minus the strength of said second beam is less than orequal to a threshold amount greater than or equal to zero; and means forenabling the communication station to transmit traffic to the userterminal on said beams within said new active beam set.
 19. The systemof claim 18, wherein said means for enabling the communication stationto determine, based on said plurality of beam strength values, a newactive beam set further comprises: (f) means for enabling thecommunication station to select a second subset of beam strength valuesfrom said first subset, wherein each beam strength value within saidsecond subset of beam strength values corresponds to an active beam,wherein an active beam is a beam over which information is currentlybeing transmitted between the user terminal and the communicationstation; (g) means for enabling the communication station to select athird beam having the strongest beam strength based on said secondsubset of beam strength values; and (h) means for enabling thecommunication station to include said third beam in said new active beamset, wherein the communication station includes said third beam in saidnew active beam set if the strength of said first beam minus thestrength of said third beam is less than or equal to a second thresholdamount.
 20. The system of claim 19, wherein said second threshold amountis greater than or equal to said first threshold amount.